Oxidation of aromatic hydrocarbons



United States Patent OXIDATION OF AROMATIC HYDROCARBONS George C. Feighner and Russell G. Rose, Ponca City,

Okla., and Paul A. Lobo, Scheveningen, Netherlands,

assignors to Continental Oil Company, Ponca City,

Okla, a corporation of Delaware No Drawing. Filed May 7, 1964, Ser. No. 365,816

9 Claims. (Cl. 260-524) The present application is a continuation-in-part of application Ser. No. 106,529, filed May 1, 1961 and now abandoned.

This invention rel-ates to the oxidation of aromatic hydrocarbons to carboxylic acids. More particularly, it relates to the oxidation of alkyl-substituted aromatic hydrocarbons to the corresponding carboxylic acids by means of molecular oxygen in the presence of a catalyst.

Numerous methods have been suggested in the prior art as possible methods for the oxidation of such hydrocarbons. All, however, possess certain disadvantages. Only a few methods possess features which render them commercially feasible. Many of the proposed methods employ chemical oxidizing reagents, such as nitric acid, sodium or potassium chromates or dichromates, potassium permanganate, and others. Obviously, methods utilizing chemical oxidizing agents for the oxidation of such hydrocarbons are expensive because of the chemicals consumed. Other methods have employed oxygen or air as the oxidizing agent. Such methods usually employ an oxygen carrier. An example of such a method is US. Patent 2,120,672, issued June 14, 1938, to J. -R. Mares, which teaches a process for the production of benzoic acid by oxidizing aromatic hydrocarbons in the presence of caustic soda and an alkali chromate as an oxygen carrier.

It is, therefore, a principal object of our invention to provide an efiicient process for the oxidation of alkylsubstituted aromatic hydrocarbons to the corresponding carboxylic acids, which process obviates the disadvantages of the prior art. Other objects and advantages of the present invention will be apparent to those skilled in the art as the invention is more fully described.

Broadly stated, the present invention provides a catalytic process for the oxidation of alkyl-substituted aromatic hydrocarbons to the corresponding carboxylic acids, which comprises intimately contacting said alkyl-substituted' aromatic hydrocarbons with a gas containing free oxygen at superatmospheric pressures and elevated temperatures in the presence of aqueous base and a novel catalyst comprising a metal halide.

Having stated the nature of our invention, it may be well to indicate wherein our invention possesses advantages as compared to the process taught by US. Patent No. 2,120,672. The metal halides used in our process are Water-soluble and cause no handling problems, as do the chromates. In addition, the metal halides are relatively inexpensive, as compared to the alkali chromates.

Before proceeding with specific examples illustrating our invention, it might be better, first, to define the materials used and the more important operating conditions of our invention.

Suitable alkyl-substituted aromatic hydrocarbons for use in our process include alkyl-substituted benzenes and alkyl-substituted polycyclic aromatic hydrocarbons. Examples of suitable polycyclic aromatic hydrocarbons are the alkyl-substituted naphthalenes, anthracenes, phenanthrenes, naphthacenes, chrysenes, pyrenes, indenes, and fluorenes. Preferred alkyl-substituted aromatic hydrocarbons for use in our process are the alkylbenzenes and alkylnaphthalenes having one to three alkyl groups con- ICC taining from one to three carbon atoms in each group. Examples of preferred materials are the following:

Toluene Dimethylnaphthalenes Xylenes Trimethylnaphthalenes Trimethylbenzenes Ethylnaphthalene Ethylbenzene Diethylnaphthalenes Diethylbenzenes Triethylnaphthalenes Triethylbenzenes Propylnaphthalene Propylbenzene Isipropylnaphthalene Cumene Dipropylnaphthalenes Dipropylbenzenes Diisopropylnaphthalenes Tripropylbenzenes Tripropylnaphthalenes Triisopropylbenzenes Triisopropylnaphthalenes Methylnaphthalene Particularly preferred materials are the alkylbenzenes having one or two alkyl groups containing from one to three carbon atoms in each group. Of the particularly preferred materials, the xylenes are especially preferred. The term xylene or xylenes refers to the meta, ortho, and para isomers, and mixtures thereof. In addition, it refers to commercial xylene which is a mixture of the three isomers with a minor amount of ethylbenzene being present.

It should be noted that, under the conditions of our invention, complete conversion of xylene to the corresponding phthalic acid does not occur, there being some toluic acid present. Any attempt to convert completely to the phthalic acid results in the production of abnormal amounts of carbon dioxide. Consequently, we have found that it is better to operate under milder conditions and to recycle the unreacted xylene and toluic acid. It should be emphasized, however, that our invention gives an improvement in yield of phthalic acid and reduces the amount of material which must be recycled. The term phthalic acid refers to any of the isomers of benzenedicarboxylic acid, which are known commonly as phthalic acid, isophthalic acid, and terephthalic acid. Where ethylbenzene is present in the material being oxidized, the product will contain benzoic acid.

Pressures wthin the range of 375 to 2500 p.s.i.g. and temperatures within the range of 225 to 325 C. can be used in our process.

Any inorganic base or inorganic basic compound can be used in our process provided it has some degree of solubility, even though slight, in water. Examples of suitable aqueous bases include aqueous solutions of hydroxides of the following metals: lithium, sodium, potassium, rubidium, cesium, calcium, strontium, barium, and zinc. Examples of suitable basic compounds include carbonates of the following metals: lithium, sodium, potassium, rubidium, cesium, and zinc. In addition to the preceding, ammonium hydroxide is suitable for use in our process. The preferred bases and basic compounds are the alkali metal hydroxides and alkali metal carbonates which should be in an aqueous solution.

The total amount and concentration can be varied within wide limits, with the only requirement being that the reaction mixture must be non-acidic at all times. Stated in another way, the amount of aqueous base must be suflicient to provide that the eflluent from the reactor is in the neutral or alkaline range. A suitable pH range for the efiluent from the reactor is from about 6 to about 10, with a preferable range being from about 7 to about 8. These operating conditions are not related to the salient features of 'our invention. Our invention will show an improvement within any of the ranges of operating conditions hereinbefore noted.

Commercially available pure oxygen is the preferred source of oxygen for use in our process. Alternatively, if

O desired, air or a mixture of oxygen and nitrogen can be used as a source for the oxygen used in the process. Again, this does not form a salient feature of our invention.

The metal halides, which are suitable for our process, maybe characterized as follows: (a) the halide ion can be chlorine, bromine, or iodine, and (b) the metal can be lithium, sodium, potassium, magnesium, calcium, barium, or aluminum. The amount of metal halide which can be used varies from 0.001 to 0.50 part per part of hydrocarbon used. Preferably, we use from 0.01 to 0.14 part of metal halide per part of hydrocarbon.

It will be readily apparent to those skilled in the art that many uses exist for the acids produced by our process. For example, they can be used to prepare polyesters, plasticizers, and synthetic fibers.

In order to disclose more clearly the nature of the present invention and the advantages thereof, reference will hereinafter be made to certain specific embodiments which illustrate the flexibility of the herein-described process. It should be clearly understood, however, that this is done solely by way of example and is not to be construed as a limitation upon the spirit and scope of the appended claims.

' Examples I-VIII The following conditions were constant in these examples:

Charge: 66.8 grams p-xylene; 320 grams of 25% NaOH solution (aqueous) Time: 1 hour The conditions which were varied are shown in Table I.

Prcedure.A stirred, nickel autoclave was charged with the xylene and aqueous caustic, sealed, and heated to 240 C. In Examples II and IV-VIII a known amount of metal halide was added to the reactants. Oxygen was admitted at a rate such as to keep the oxygen partial pressure at the desired value. The duration of the run was one hour. After cooling and releasing the pressure, the product was removed from the autoclave. The unreacted xylene was recovered, and the acids were isolated by acidification, filtering, and drying. The CO was caught in an Ascarite trap.

The data on the products obtained are shown in Table Example II, as compared to Example I, shows no significant improvement in yield of total acids but a substan tial improvement in the yield of terephthalic acid. Examples IV, VI, VII, and VIII, as compared to Example 111, show improvement in both yield of total acids and yield of terephthalic acid. Example V, as compared to Example III, shows a substantial increase in yield of total acids with a decrease in percentage yield of terephthalic acid. It should be noted that the net yield. of terephthalic acid in Example V is about twice that of Example III. From the data of these examples, it is readily apparent that the use of a metal halide gives a substantial improvement in the yield of terephthalic acid.

While the examples'given have dealt only with batch operation, our invention is not limited thereto. The process of our invention will work in a continuous process as well as in a batch process.

While particular embodiments of the invention have been described, it will be understood, of course, that the invention is not limited thereto, since many modifications may be made; and it is, therefore, contemplated to cover by the appended claims any such modifications as fall within the true spirit and scope of the invention.

The invention having thus been described, what is claimed and desired to be secured by Letters Patent is:

1. A process for the oxidation of xylenes, to the corresponding phthalic acids, said process comprising contacting said xylenes with a gas containing free oxygen under a pressure of about 375 to 2500 p.s.i.g. and a temperature of about 225 to 325 C. in the presence of an aqueous base and a metal halide wherein the halide anion .is

selected from the group consisting of chloride, iodide and present in an amount in the range of 0.01 to 0.14 part Y per part of xylenes.

TABLE I.REACTION CONDITIONS-EXAMPLES I-VIII Example Weight of Catalyst/ Tempera- Total Partial No. Catalyst Catalyst, g. p-Xylene ture, 0. Pressure, Pressure Ratio p.s.i.g. O2, p.s.i.g.

T Nnne 240 665 II NaBr- 0.9 O. 013 240 700 None 275 1, 125 125 NaCl 9.0 0. 275 1,140 7 150 NaI 0.9 0. 013 275 1, 130 NaBr 0.9 0.013 275 1, 110 170 OaClg 0. 9 0.013 275 1, 120 VIII AlCl;- 0. 9 0. 013 275 l, 130

TABLE II.PRODUCT YIELD AND ANALYSISEXAMPLE I-VIII Analysis of Acids Example CO2, Acid, Terephthalic No. Catalyst Grams Grams Acid, Grams Terephthalic, pTolu1c,

Percent Percent 1. 1 11. 1 30. 1 69. 9 3. 34 2. 3 1l. 6 47. 6 '52. 4 5. 52 11. 4 25. 8 59.6 40. 4 15.38 12. 8 46. 0 64. 3 35. 7 29. 58 9. 7. 57.1 54. 6 45. 4 31.18 10. 7 33.0 65. 3 34. 7 21. 55 12. 0 84. 5 62. 8 37. 2 21. 67 VIII A1013- 11.3 37. 8 61. 2 38. 8 23.13

6. The process of claim 5 wherein the aqueous base References Cited is selected from the group consisting of alkali metal UNITED STATES PATENTS hydroxides and alkali metal carbonates.

7. The process of claim 6 wherein the halide anion is 2907792 10/1959 McIntyre 260 524 ch1oride 5 FOREIGN PATENTS 8. The process of claim 6 wherein the halide anion is 808,581 2/1959 Great Britain. iodide.

9. The process of claim 6 wherein the halide anion LORRAINE WEINBERGER Primary Examineris bromide. SIDNEY B. WILLIAMS, IR., Assistant Examiner. 

1. A PROCESS FOR THE OXIDATION OF XYLENES, TO THE CORRESPONDING PHTHALIC ACIDS, SAID PROCESS COMPRISING CONTACTING SAID XYLENES WITH A GAS CONTAINING FREE OXYGEN UNDER A PRESSURE OF ABOUT 375 TO 2500 P.S.I.G. AND A TEMPERATURE OF ABOUT 225 TO 325* C. IN THE PRESENCE OF AN AQUEOUS BASE AND A METAL HALIDE WHEREIN THE HALIDE ANION IS SELECTED FROM THE GROUP CONSISTING OF CHLORIDE, IODIDE AND BROMIDE AND THE CATION IS SELECTED FROM THE GROUP CONSISTING OF LITHIUM, SODIUM, POTASSIUM, MAGNESIUM, CALCIUM, BARIUM, AND ALUMINUM, SAID METAL HALIDE BEING PRESENT IN AN AMOUNT IN THE RANGE OF 0.002 TO 0.50 PART PER PART OF XYLENES. 